BACKGROUND: The energy consumed per covered distance (C) is increased in hemiparetic stroke adults during walking. OBJECTIVE: To ascertain if increased C in stroke patients is a result of increased mechanical work, of decreased efficiency of work production by muscles or of slow walking speed. METHODS: C and mechanical work were computed in 20 patients walking on a force measuring treadmill at speeds ranging from 1kmh(-1) to their own maximum speed (WS(MAX)). Works done by healthy and pathological limbs were computed separately. RESULTS: For hemiparetic patients, C was around 1.7 times greater than normal. When these patients had a slower WS(MAX), they had greater C and mechanical work (r=-0.44 and -0.57, respectively). The increased C was related to the external work performed to lift the center of body mass when the healthy limb was supporting the body weight (r=0.77). CONCLUSIONS: The increase of C in stroke patients is more pronounced when WS(MAX) is slow. Moreover, this increase is related to increased mechanical work done by muscles and is not related to slow walking speed or decreased efficiency. As in healthy subjects, C and external work presented optimum speeds, indicating a preserved pendular mechanism of walking.